Clone Brush doesn't paint seam
Are you using Subsurf? If you are, do you have Subdivide UVs checked? Apparently this can cause seams to be exposed.
https://developer.blender.org/T26426
Try unchecking Subdivide UVs and see if it helps.

Also adding loops at the edges might help: https://blenderartists.org/forum/archive/index.php/t-343226.html